Cadmium sulfide (CdS)-type photovoltaic cells have shown considerable promise for terrestrial photovoltaic applications. Depositing the CdS-type layer by spray pyrolysis appears to provide a process which is adaptable to low cost and large scale production of photovoltaic cells. Using the process described by Jordan and Lampkin in U.S. Pat. Nos. 3,880,633, 3,902,920, and 4,159,914, the spray process can produce very thin films of CdS in the order of 2-4 microns in thickness.
In order to produce a photovoltaic cell, a portion of the CdS layer is converted to cuprous sulfide (Cu.sub.x S) to form the heterojunction, or barrier layer, where the energy conversion occurs. It is generally taught in the prior art that forming the Cu.sub.x S layer is preferably accomplished by dipping the CdS film into a solution of copper ions, where the conversion is obtained by an ion conversion process. The dip solution can penetrate along CdS grain boundaries and convert sufficient CdS wherein acceptable open circuit voltages (Voc) and short circuit currents (Isc) are obtained. However, the dip solution can penetrate through the entire CdS layer along grain boundaries or through pin-hole defects in the CdS layer, wherein the CdS layer is short circuited and the photovoltaic effect is inoperative.
One solution proposed in U.S. Pat. No. 4,159,914 is to provide a bi-layered CdS structure. The first layer of CdS is formed on a transparent electrically conductive substrate and is formed with a crystal structure which resists penetration of the dip solution. The first layer of CdS is generally formed with small sized crystals, less than 0.10 microns, and may be amorphous in appearance. The second CdS layer is conventionally formed of CdS crystals having dimensions in the range of 0.1-1.0 microns. The bi-layered CdS cell permits a process yielding a high percentage of operable photovoltaic cells.
However, the bi-layered cell obtains only relatively low Voc. Although current densities of up to 26 ma/cm.sup.2 have been obtained from the bi-layered cell, which is close to a theoretical limit, maximum Voc of only about 440-460 mv have been observed. It is theorized that the penetrating dip solution forms a heterojunction with the small crystals in the first CdS layer along the interface between the two layers. The Voc of a single crystal is believed to be a function of the crystal size, wherein the small first layer crystals produce relatively small open circuit voltage. Thus, the first layer open circuit voltage would be less than the second layer open circuit voltage, wherein the overall cell open circuit voltage would be reduced.
One solution to the low open circuit voltage problem is to use a weak dip solution, where copper ions do not penetrate to the interface to form heterojunctions with the first layer crystals. Experiments have shown that a higher voltage does, indeed, result. However, the resulting Cu.sub.x S layer formed on second layer crystals is not sufficiently deep to capture the incident radiation. This results in a reduced current production. Hence, no overall gain in cell performance is obtained.
Yet another solution is to form a single layer of CdS to a thickness sufficient to permit grain boundary penetration without shorting through the CdS layer. This, of course, would be unacceptable for a backwall configuration CdS cell where incident radiation must pass through the CdS layer to the heterojunction. Even in a front wall configuration, the resulting thickness of CdS would require 5-10 times the CdS thickness needed for photon capture, resulting in a waste of scarce cadmium resources and an increase in overall cell production costs.
These disadvantages of the prior art are overcome by the present invention, however, and an improved fabrication process is provided for forming a CdS layer from a spray process and forming a heterojunction from evaporated copper-containing materials, wherein large CdS crystals are produced on which the heterojunctions are formed to yield improved open circuit voltages and increased photovoltaic conversion efficiencies.